1. Technical Field of the Invention
The present invention relates to novel topically applicable cosmetic and/or dermatological compositions well suited for the photoprotection of the skin and/or the hair against ultraviolet radiation (referred to hereinbelow more simply as "screening" or "sunscreen" compositions) and to a general technique for the photostabilization of specific UV-A-active sunscreens by means of particular polysaccharide alkyl ethers.
More especially, this invention relates to UV-photostable screening compositions which comprise, in a topically applicable, cosmetically and/or dermatologically acceptable vehicle, diluent or carrier, at least one dibenzoylmethane compound active as a UV-A organic sunscreen, together with a particular polysaccharide alkyl ether as a photostabilizing agent therefor, and to a corresponding process for the stabilization of such at least one dibenzoylmethane compound by means of said polysaccharide alkyl ether.
2. Description of the Prior Art
It is known that light radiation of wavelengths of from 280 nm to 400 nm promotes tanning of the human epidermis, and that light rays of wavelengths of from 280 nm to 320 nm, i.e., UV-B irradiation, cause skin burns and erythema which may be harmful to the natural development of the tan; hence, such UV-B radiation should be screened from the skin.
It is also known to this art that UV-A radiation, of wavelengths of from 320 nm to 400 nm, which causes tanning of the skin, also adversely affects it, especially in the case of sensitive skin or of skin which is continually exposed to solar radiation. UV-A irradiation causes, in particular, a loss in the elasticity of the skin and the appearance of wrinkles, promoting premature skin aging. Such irradiation promotes triggering of the erythemal reaction or amplifies this reaction in certain individuals and may even be the cause of phototoxic or photoallergic reactions. Thus, it is desirable to also screen out UV-A radiation.
In this respect, one particularly advantageous class of UV-A screening agents currently in use comprises dibenzoylmethane derivatives, and in particular 4-(tert-butyl)-4'-methoxydibenzoylmethane, which exhibit a high intrinsic power of absorption. These dibenzoylmethane derivatives, which are compounds that are well known per se as UV-A-active screening agents, are described in FR-A-2,326,405 and FR-A-2,440,933, as well as in EP-A-0,114,607; 4-(tert-butyl)-4'-methoxydibenzoylmethane is moreover currently commercially available under the trademark "Parsol 1789" from Givaudan.
Unfortunately, these dibenzoylmethane derivatives are compounds which are relatively sensitive to ultraviolet radiation (especially UV-A), namely, more precisely, they have an unfortunate tendency to degrade relatively rapidly under the action of ultraviolet radiation. Hence, this substantial lack of photochemical stability of dibenzoylmethane derivatives towards ultraviolet radiation, to which they are by nature intended to be subjected, does not make it possible to guarantee constant protection during prolonged exposure to the sun, such that the user must make repeated applications at close and regular time intervals in order to obtain effective protection of the skin against UV irradiation, which obviously is a disadvantage.
To date, the photostabilization of dibenzoylmethane derivatives with respect to UV radiation remains a problem which has not yet been solved entirely satisfactorily.